The compound ammonium sulfate is a byproduct of several industrial processes. One such process is the production of the chemical hydroquinone from the hydrolysis of p-aminophenol by ammonium bisulfate (Greco, U.S. Pat. No. 3,862,247). Another such process is the production of the chemical methyl resorcinol from the hydrolysis of toluene diamine by ammonium bisulfate (Greco, U.S. Pat. No. 3,933,925). Ammonium sulfate is also a byproduct in the production of caprolactam, which is a monomer for nylon-6 production (Bonfield, U.S. Pat. No. 3,282,646).
Another process that produces an ammonium sulfate byproduct is the production of biofuels or chemicals from cellulose-containing feedstocks, such as agricultural wastes, grasses, and forestry wastes. The most common process for producing biofuels or chemicals from cellulose involves breaking down the fibrous material with pretreatment, followed by a nearly complete conversion of the cellulose to glucose by using cellulase enzymes. In the pretreatment process, steam and sulfuric acid are typically used to hydrolyze the hemicellulose to xylose, arabinose, mannose, galactose, acetic acid, glucuronic acid, formic acid, and galacturonic acid. The pretreatment does not hydrolyze a large portion of the cellulose, but rather increases the cellulose surface area as the fibrous feedstock is converted to a muddy texture. The pretreated cellulose is then hydrolyzed to glucose in a subsequent step of enzymatic hydrolysis that uses cellulase enzymes. The glucose is then fermented by yeast or bacteria to produce biofuels such as ethanol or butanol, or chemicals such as lactic acid.
Prior to the addition of cellulase enzymes to the pretreated feedstock, the pH of the acidic feedstock is adjusted to a value that is suitable for the enzymatic hydrolysis reaction. Typically, this involves the addition of alkali to a pH of between about 4 to about 6, which is the optimal pH range for cellulases, although the pH can be higher if alkalophilic cellulases are used. The pH can, in principle, be adjusted with any base, but, in practice, ammonium hydroxide is the most desirable. The adjustment of the pH with ammonia or ammonium hydroxide produces ammonium sulfate salt. Alternatively, ammonia or ammonium hydroxide can neutralize sulfuric acid in other steps of the process, thereby producing ammonium sulfate.
An alternative pretreatment is ammonia fiber explosion (often known as AFEX) in which the feedstock is subjected to concentrated ammonia at high pressure. The ammonia is released quickly by explosive decompression and recovered. The ammonia residue within the pretreated fiber results in ammonium sulfate when sulfuric acid is used to adjust the pH to 4 to 6 prior to enzymatic hydrolysis.
Ammonium sulfate is an important chemical in the fertilizer industry. In some geographic regions, an ammonium sulfate byproduct from a chemical process can be sold as fertilizer. However, in regions where ammonium sulfate fertilizer usage is limited, excess ammonium sulfate must be disposed of. In such cases, it is much more desirable to recover ammonia and sulfuric acid from the ammonium sulfate.
Greco (U.S. Pat. Nos. 3,862,247 and 3,933,925) describes the conversion of ammonium sulfate to ammonia and ammonium bisulfate. This is carried out by heating ammonium sulfate to a temperature of 310° C. to 450° C. At temperatures above 450° C., the ammonium bisulfate decomposes. After heating for a few minutes at 330° C., 75% to 95% of the ammonium sulfate is converted to ammonium bisulfate. The ammonia is recovered and used in other processes. Greco's process recovers half of the ammonia in the ammonium sulfate, and produces ammonium bisulfate. However, the process does not recover sulfuric acid and the ammonia yield is relatively low (50%).
Halstead (J. Appl. Chem., 1970, 20:129-132) describes the decomposition of ammonium sulfate at 400° C. The first reaction converts ammonium sulfate to ammonium bisulfate and ammonia. In a second reaction, the ammonium bisulfate dehydrates to form a water molecule and ammonium pyrosulfate, (NH4)2S2O7. Further heating of the ammonium pyrosulfate to form sulfur dioxide and nitrogen, which are not desired products from ammonium sulfate, is also carried out.
D. J. LeCaptain (Central Michigan University), reports the conversion of ammonium sulfate to ammonia and ammonium bisulfate by heating. The process described (see: http://72.14.253.104/search?q=cache:h6w1_ixVZNEJ:www.cst.cmich.edu/units/chm/people/D_Lecaptain.htm+cst.cmich.edu/units/chm/people/D_Lecaptain.htm&hl=en&ct=clnk&cd=1&gl=ca) does not recover more than 50% of the ammonia, and does not recover sulfuric acid.
U.S. Pat. No. 3,282,646 (Bonfield) discloses the production of ammonia and sulfur dioxide from ammonium sulfate. The ammonium sulfate was heated to 250° C. to drive off ammonia and produce ammonium bisulfate. The temperature was increased to 450° C. and carbon monoxide, hydrogen sulfide, hydrogen, or nitrogen was bubbled through the molten salt. The reaction produced ammonia and sulfur dioxide, the latter being an undesirable byproduct.
Kiyoura and Urano (Ind. Eng. Chem. Process Des. Develop., 1970, 9(4):489-494) describes the thermal decomposition of ammonium sulfate to ammonia, sulfur dioxide, sulfur trioxide and other gases. Kiyoura states that the simple decomposition of ammonium bisulfate to sulfuric acid and ammonia is not an adequate mechanism to describe the thermal decomposition reaction.
Liske et al. (Journal of Hazardous Substance Research, 2000, 2:8.1-8.17) teach the complete thermal decomposition of ammonium sulfate to several gaseous products. This combustion process does not recover ammonia and sulfuric acid.
Dugger et al. (Ammonium Sulfate Decomposition, 1955, RMO-2036, United States Atomic Energy Commission) describes the thermal decomposition of ammonium sulfate in the presence of zinc oxide. The reaction products are ammonia and sulfur dioxide.
U.S. patent application 20040234441 (Hansen) describes heating a mixture of ammonium sulfate and sulfuric acid in a ratio of 1:2 to 285° C. to decompose the mixture to ammonia and sulfuric acid. The temperature is maintained below 290° C. to avoid boiling of the sulfuric acid. A disadvantage of this process is that the temperature must be controlled in such a narrow range.
U.S. Pat. No. 4,081,515 (Gruhier) carries out the decomposition of ammonium bisulfate by heating it to 400° C. The heating is carried out without a catalyst present or with copper, molybdenum, or tungsten catalyst present. The primary product is sulfur dioxide, which is not desired in the recovery of ammonia and sulfuric acid.
U.S. Pat. No. 4,490,347 (Gelblum) describes the production of oleum (sulfuric acid containing sulfur trioxide) from a mixture of ammonium bisulfate and sulfuric acid, by pyrolyzing the mixture in the presence of oxygen. The production of oleum is not desirable in the recovery of ammonia and sulfuric acid from ammonium sulfate.
Huter (German Patent 1,151,492) describes the production of ammonia and sulfuric acid from ammonium sulfate or ammonium bisulfate. The process starts by adding potassium sulfate to ammonium bisulfate in a 1:1 molar ratio and heating the mixture at 350° C. This drives off the ammonia and produces potassium bisulfate. The potassium bisulfate is then heated in a second stage to a temperature range between 600° C. and 650° C. to liberate sulfur trioxide, which is combined with water to make sulfuric acid, and potassium sulfate. Although Huter's process produces ammonia and sulfuric acid from ammonium sulfate, the high temperature of the second stage reaction requires a large amount of energy, and the sulfur trioxide and any ammonium sulfate or ammonium bisulfate carried through to this point are unstable. These disadvantages have limited the adoption of Huter's process.
Therefore, a satisfactory process for the recovery of ammonia and sulfuric acid from ammonium sulfate is lacking. The ability to recover these compounds from ammonium sulfate represents a large opportunity to avoid the cost of disposal of ammonium sulfate and to lower process costs by reusing or selling ammonia and sulfuric acid.